The topic of the present study is the establishment of a highly sensitive and accurate method for detection of the structural modification of DNA caused by covalent and non-covalent interactions with benzo(a)-pyrene (BP) metabolites. The effects of carcinogens bound to DNA on the functions of that DNA were studied by assaying replication, mutation, and repair of the DNA using both an in vitro replication system and a unique mutation assay system developed in this laboratory. Several different clones of plasmid DNA which were mutated by a small number of BP metabolites are selected. Their transformability, restriction endonuclease map, and DNA base sequence are currently being studied in order to determine the location and chemical nature of DNA modification.